This is a fascinating puzzle, and completely new to me — the moving rocksof Racetrack Playa in the Panamint Mountains in Death Valley National Park, California. The moving rocks, also called sailing stones or sliding rocks, slide across the surface of the playa leaving long tracks behind them. A playa is a seasonally dry lake bed. No one has ever seen the stones move, but the trails indicate that they do. Nor are these all little rocks, many are as big as a man, and some are as large as 750 lbs. For more pictures and possible explanation, follow the link above.